The prior art of safety electrical socket and connector design includes many electrical wall outlets and light bulb sockets constructed for tamper-proof and shock-proof operation. A common feature of the prior art designs is the provision of a mechanical internal switch, generally operated upon insertion of an electrical plug or electrical load device, such as a light bulb, into the socket or receptacle. Operation of the mechanical switch completes the electrical connection of the electrical plug or load device with live contacts to which power is continuously applied. Examples of this design can be found in the following U.S. Patents:
U.S. Pat. No. 4,271,337 to Barkas; PA0 U.S. Pat. No. 4,179,175 to Farnworth et al.; PA0 U.S. Pat. No. 4,152,557 to Busch et al.; PA0 U.S. Pat. No. 4,148,536 to Petropoulsos et al.; PA0 U.S. Pat. No. 4,093,336 to Rose; PA0 U.S. Pat. No. 3,915,536 to Glantz; PA0 U.S. Pat. No. 3,895,195 to Morrison et al.; PA0 U.S. Pat. No. 3,699,285 to Leatherman; PA0 U.S. Pat. No. 3,596,019 to Koester; PA0 U.S. Pat. No. 3,579,171 to Woodward; and PA0 U.S. Pat. No. 2,735,906 to Avrunin. PA0 (a) it has limited sensitivity, and is dependent on a critical calibration setting; PA0 (b) it operates on stored spring energy, and with the passage of time, friction, dirt, and ageing of mechanical parts, reliability is reduced; and PA0 (c) it has a slow response since it depends on mechanical amplification which is accomplished in several stages which proceed in serial fashion.
Another design also intended for shock-proof operation is that disclosed by U.S. Pat. No. 4,647,120 to Karabakakis, in which an electrical safety plug carries a small magnet which magnetically activates an internal switch within a socket upon insertion. A variation of this design is disclosed in U.S. Pat. No. 4,616,285 to Sackett in which the magnetically operated switch is accessible from outside the socket.
In all of the above outlined prior art designs, the attempt to limit the risk of electrocution is based on mechanical barriers, actuators, etc. which attempt to prevent a person from touching live contacts. Because they are based on mechanical designs, they can only limit the risk of electrocution but do not eliminate it since they are subject to mechanical failure, corrosion, wear and tear, and the possibility of tampering to defeat their function. If mechanical or actuation failure is encountered in any of these designs, the results may be disastrous, since the electrical socket contacts are always energized by the power distribution system, and these may become accessible to human touch.
Another electrical safety device in widespread use on 220 volt-single phase residential power systems is a differential circuit breaker, designed to prevent the risk of electrocution. An example of this device is known as a "Schutzschalter" (also known as an earth leakage circuit breaker) which is manufactured and marketed by Siemens AG of Germany. This device contains a toroid, which is wound with a phase, neutral and pickup winding. The device operates on the principle of imbalance in the current between phase and neutral, which leads to induced electromagnetic flux in the toroid, and which can be detected by a pickup winding. The pickup winding is connected to a tiny solenoid which pulls a lever and triggers a mechanical arrangement for operating the breaker.
Since the phase and neutral windings usually do not produce exactly the same electromagnetic flux, there is always a small difference which is detected by the pickup winding, producing an induced voltage. To compensate for this problem while achieving maximum sensitivity, a small magnet is provided which can be calibrated by rotation so as to exert a counterforce on the trigger lever of the mechanical arrangement which operates the breaker. However, the calibration must be done at the factory and since it is a force-balance device, its calibration tolerance is critical.
Typically, the rated circuit breaker protection is for 30 mA leakage current. This leakage current trip rating is dangerous in the case of the human body, which will experience quite a shock at this level. If, however, the sensitivity is increased for response to smaller leakage current levels, the device will malfunction. This is because usually the device is installed to protect the entire household, and since it must serve many loads each of which has an inherent small leakage current, increasing its sensitivity may cause false tripping.
Another problem with the differential circuit breaker device is the design of the tripping mechanism, which requires mechanical amplification to permit the small solenoid to break high current contacts. The common approach is to use a "mechanical amplifier" which operates based on stored spring energy, and it has been reported that 30% of these devices do not function after one year in use.
Still another problem with this device is the speed of response. Because it is a mechanical design, the best expected response is in the tens of milliseconds range, and this exposes a person to an electrical shock, although it may not be fatal.
As outlined above, the differential circuit breaker device suffers from at least three inadequacies:
Therefore, it would be desirable to provide a safety electrical socket which does not depend on the operation of mechanical parts, and therefore achieves a greater level of reliability and safety in assuring substantial elimination of electrocution risks.